This invention relates to polymer surfaces and in particular to their reaction with organic molecules to provide a functionalized surface. In particular, it relates to a new method of grafting maleic anhydride (MAH) to the surface of a polyolefin.
Polyolefins are well known industrially for their chemically inert surfaces. As an example, polypropylene, one of the most commonly used polyolefins in commercial applications, has a typical surface energy of approximately 30-40 dynes/cm and bonding or coating requires 50 dynes/cm and above.
There are a number of bulk properties that make polyolefins such as polypropylene or polyethylene desirable for use in the car parts industry. Uses in this industry sector include bumpers, fascia, interior components, seals and weather stripping. Polypropylene is also used in the manufacture of compression moulded parts for various applications, such as toys. It is also spun into fibres for the manufacture of clothing.
The increased use of polypropylene and other polyolefins is related to the low material cost, excellent mechanical properties, recyclability and excellent processability.
The applications for specific surface modification of polyolefins are many and diverse. They can include modification to allow or disallow interactions between the layers of polymers, addition of surface groups to provide a conductive surface and modification of biomedical resins to facilitate biological reactions or separations.
In many instances it is desirable to have specific functionality at the surface of a material to provide improved bonding, an antibacterial barrier, a reactive group for further manipulation or a surface with a specific reactivity. These surfaces may be in the form of films, moulded parts, fibres or powders.
In applications where one would like oxygen functionality at the surface of a polyolefin such as polypropylene, without sacrificing any of the bulk properties, a number of treatments have been well investigated. These include corona discharge, flame, glow discharge plasma, chemical priming and ultraviolet light in the presence of ozone--all well-known in the art.
These techniques work satisfactorily well but tend to result in a mixture of oxidized functionalities at the surface.
One common process currently employed in industry arose from the desire to bond polypropylene to a metal surface. This process involves adding an organic molecule, such as maleic anhydride (MAH), to bulk polymer to promote adhesion between the polyolefin and other polymer or metal surfaces. One present industrial method to accomplish this is to reactively co-extrude the polymer premixed with fairly high levels of the organic molecule in the presence of a peroxide radical to initiate the reaction.
Problems with this method include incomplete incorporation of the organic molecule as well as extensive cross linking or chain scissioning of the polymer. The cross linking increases the viscosity of the polymer and increases the residence time in the reactor. The increased time in the reactor can lead to many side reactions and an undesirable product.
Although resin made by this method works satisfactorily in forming polymer-to-metal and polymer-to polymer bonds, the bulk of the resin contains the reactive molecule even though only the top few layers of atoms is needed to provide the bond. This current process thus makes ineffective use of the organic molecule. It also uses excessive reagent and modifies the bulk properties of the resin to the extent that loss of the mechanical properties and a decrease in processability can occur.
The problems that are encountered with the use of excessive amounts of modifying organic molecules also include phase separation, where the initiator dissolves in the organic component and causes it to react with itself before it can react with the resin. There are also problems encountered with incompatibilities between the polymer and the organic molecule in the extrusion. The temperatures in the extrusion barrel must be monitored closely to provide optimum reactor conditions.
This method is also limited in the types of molecules which can be incorporated.